I wouldn't do it right now. Kids that have been trained for a really long time will sometimes regress when a new baby comes, so it will likely be a waste of your time to do it right now, especially if she doesn't want to. My daughter was a very bright 2 1/2 year old also when my son was born. I didn't train her until after he was born and she insisted on wearing panties (when he was a month or so old). She trained fairly quickly and was fully trained for several months and then decided that she wasn't going to poop in the potty anymore. It was so frustrating because she hadn't had an accident in MONTHS, so I knew she knew what she was doing. I eventually found an incentive that worked for her and we got it back under control, but it was maddening.
Now, my son is 3 years and 3 months old and I have just potty trained him in the past few weeks. He just flat wanted nothing to do with it, but he is well over 3 and very smart also, just strong willed and lazy (why stop to go potty when Mom will keep cleaning me up?). I wanted him to start preschool and none of the schools here would take a 3 year old that wasn't potty trained, so I just put him in undies and told him no more diapers. He didn't do great at first, but after a few weeks, totally gets it now. That doesn't mean he doesn't still have accidents (he will still poop in his pants occassionally if he's tired or in a foul mood and is just being stubborn), but he knows what to do now and the accidents are few and far between. He even does great at school now and hasn't had an accident there in 2 weeks (he is only there 2 days per week).
Anyway, all of that to say that with some kids, I think you're just going to have to do it at some point because they will never choose to do it on their own, but at 2 1/2 with a new sibling coming along in a couple of months, it will likely be an uphill battle for you. I suggest waiting until the baby is born and then playing up the "babies, like your sister/brother wear diapers. Big kids wear panties...want to try to wear panties like a big kid?" and see if she will go along with it then. I wish you much luck. Potty training is so frustrating and messy! But, I'm happy that this will be my last experience with it and (not to jinx myself) I think we're just about there! :-)